Answer: Uuencode

Long before there were discrete email attachments and the MIME format that made such attachments ubiquitous, there was uuencode. The very first email attachments were distributed using a binary-to-text protocol based on the Unix programs uuencode (Unix-to-Unix encoding) and uudecode (Unix-to-Unix decoding). Uuencode allowed users to take a binary file (such as a spreadsheet document or image file) and change its native binary format into a block of text that could be embedded into the body of an email.

In this fashion, files like images could be converted into a medium that could be inserted into a plain-text email and then retrieved and decoded on the other end by the recipient using uudecode. The process was typically manual and required the recipient to know what they were looking at (and what to do with it), but in the days before automated email attachments and a transparent attachment system, it allowed email-based file distribution with (relative) ease.

Uuencoding and uudecoding were also widely used to distribute files via the global discussion board system Usenet throughout the late 20th century where it enjoyed longer-lasting popularity than it did in the email arena (where usage tapered off rapidly after the introduction of the MIME system).